This invention relates to a rolling dies for producing dog point threads each having a thread portion provided with external threads and a dog point having a diameter smaller than that of the thread portion through a plastic deformation process by rolling an outer surface of a screw blank. More specifically, this invention relates to a thread rolling flat dies for producing dog point threads which is capable of preventing slipping of outer surface of the screw blank when rolling the screw blank, thereby avoiding breakage of external threads or the like of the thread portion.
A dog point thread has a thread portion provided with external (male) threads and a dog point of cylindrical shape provided at the end of the thread portion where a diameter of the dog point is smaller than that of the thread portion. For producing such a dog point thread, for example, a pair of thread rolling flat dies is used to plastic deform an outer surface of a work or screw blank thereby forming the thread portion and the dog point at the same time. One surface of the rolling dies has a thread portion rolling surface formed with a plurality of ridges and grooves for forming the thread portion of the dog point thread and a flat surface continuous to the thread portion rolling surface having a flat surface for forming the dog point of the dog point thread.
In the conventional thread rolling flat dies, however, since the area for forming the dog point of the dog point thread has the flat surface as noted above, the outer surface of the screw blank (work) contacting the flat surface tends to slip during the rolling. Thus, there is a problem that the slip during the rolling causes damages or irregularity of the external (male) thread formed by the rolling dies.
This invention has been made to solve the above noted problems arises in producing the dog point thread in the production method involved in the conventional technology.
It is, therefore, an object of the present invention to provide a thread rolling dies for producing dog point threads which is capable of preventing the outer surface of the screw blank from slipping during the rolling, thereby avoiding breakage of external threads or the like on the thread portion of the dog point thread.
It is another object of the present invention to provide a thread rolling dies for producing dog point threads with high production efficiency by forming a dog point thread with a rolling pressure less than a rolling pressure applied to a thread portion of the dog point thread.
It is a further object of the present invention to provide a thread rolling dies for producing dog point threads each being capable of smoothly guiding a nut or the like having internal thread toward a thread portion of the dog point thread.
In the first aspect of the present invention, the thread rolling dies is to produce a dog point thread by plastic deforming an outer surface of a screw blank where the dog point thread has a thread portion provided with external threads and a dog point having an outer diameter smaller than an outer diameter of the thread portion. The thread rolling dies is comprised of a thread portion rolling surface having a thread portion dies ridge for forming a groove of the thread portion of the dog point thread and a thread portion dies groove for forming a ridge of the thread portion of the dog point thread, and a dog point rolling surface provided in parallel with the thread rolling surface and having a dog point dies ridge for forming a groove of the dog point of the dog point thread and a dog point dies groove for forming a ridge of the dog point of the dog point thread. A crest of the dog point dies ridge is positioned higher than a crest of the thread portion dies ridge, and a root of the dog point dies groove is positioned lower than a root of the crest of the thread portion dies ridge, and a distance between the crest and root of the dog point dies ridge is smaller than a distance between the crest and root of the thread portion dies ridge.
According to the first aspect of the present invention, the dog point of the dog point thread is formed by the dog point rolling surface of the rolling dies by rolling the dog point dies ridges and dog point dies grooves on the outer surface of the work (screw blank) at the same time. In the thread rolling dies of the present invention, the crest of the dog point thread (dies) ridge is designed higher than the crest of the thread portion (dies) ridge. Further, the root of the dog point thread (dies) groove is designed lower than the crest of the thread portion (dies) ridge. Since the ridge height of the dog point thread ridge is shorter than the ridge height of the thread portion ridge as noted above, the resultant ridge height of the dog point thread is shorter than the ridge height of the thread portion ridge of the dog point thread.
In the second aspect of the thread rolling dies of the present invention for producing the dog point thread, a pitch and a lead angle of the dog point rolling surface are designed approximately equal to a pitch and a lead angle of the thread portion surface.
In the third aspect of the thread rolling dies of the present invention for producing the dog point thread, a difference of height between the crest of the dog point dies ridge and the crest of the thread portion dies ridge pitch is designed larger than about 0.05 mm and is smaller than about a half of the distance between the crest and root of the thread portion dies ridge, and a difference of height between the root of the dog point dies groove and the crest of the thread portion dies ridge is designed larger than about 0.05 mm and is smaller than about a half of the distance between the crest and root of the thread portion dies ridge.
In the fourth aspect of the present invention, the thread rolling dies for producing the dog point thread includes a cut-out groove in a boarder area between the thread portion rolling surface and the dog point rolling surface wherein the cut-out groove connects the root of the thread portion dies groove and the root of the dog point dies groove by upwardly inclining therebetween.
Therefore, the cut-out groove on the rolling dies functions to produce the boarder area between the dog point and thread portion of the dog point thread. As noted above, the cut-out groove upwardly inclines from the root of the thread portion groove to the root of the dog point groove. Thus, the dog point thread produced by the rolling dies, at the boarder area between the dog point and the thread portion, the height of the ridge of the dog point gradually increases toward the ridge of the thread portion.
In the fifth aspect of the thread rolling dies of the present invention for producing the dog point thread, width of the cut-out groove gradually decreases from the root of the thread portion dies groove to the root of the dog point dies groove.
Therefore, the cut-out groove on the rolling dies functions to produce the boarder area between the dog point and thread portion of the dog point thread. As noted above, the width of the cut-out groove gradually decreases from the root of the thread portion groove to the root of the dog point groove. Thus, the dog point thread produced by the rolling dies, at the boarder area between the dog point and the thread portion, the width of the ridge of the dog point gradually increases toward the ridge of the thread portion.